The present invention relates in general to light monitors, and will be illustrated in the context of monitoring exterior lights of vehicles such as school buses.
An ongoing challenge facing people who operate and maintain vehicles is determining when various lights in the vehicle are operating properly. For instance, in the case of a school bus the driver needs to know if the various exterior signal lights are operating properly. Ideally, the operator should know when a light fails as soon as possible so that appropriate remedial measures may be taken to fix or replace the light. Adding to the challenge of light monitoring are the wide range of electrical currents used by different lights. For instance, exterior signal lights have traditionally been incandescent lights, often using up to 10 amps or more of current per light. Increasingly, however, an array of light emitting diodes (LED) are being used in place of incandescent lights. LEDs offer a variety of advantages over incandescent lights, however, LEDs use a substantially lower current compared to incandescent lights, often less than 1 amp of current. LEDs are increasingly being retrofitted into older vehicles to replace existing incandescent lights. While a number of vehicle light monitors have been conceived, none adequately solve the problems in the manner of the present invention.
One example of the present invention is a monitor for a light circuit comprising a power source, a light being monitored, and a sense resistor. In one embodiment, the sense resistor is connected in series with the light. A first voltage amplifier circuit connected to the light circuit has a first output signal representing the voltage across the sense resistor. A second voltage amplifier circuit connected to the circuit has a second output signal representing the voltage across the sense resistor. A microprocessor is configured to receive the output signals from the first and second voltage measuring circuit and is programmed to calculate the current to the light based on the first and second digital output signals, compare the calculated current against a threshold current, and initiate a failure signal if the calculated current is less than the threshold current. An indicator activates if the microprocessor initiates a failure signal, whereby the activated indicator notifies that the light is not operating correctly.
In one embodiment, the light is an exterior light on a vehicle that is an incandescent light or a plurality of LEDs. The indicator can be an LED visible to the vehicle operator. In another embodiment, the first and second voltage amplifier circuits each comprise an analog to digital converter and the output signals are digital. The first and second digital output signals may each have a maximum value. In yet another embodiment, the calculated current is based on the first output signal if the light is a low current light, and the current is calculated is based on the second output signal if the light is a high current light. The low current can have a variety of ranges, such as being greater than about 0 amps and less than about 1 amp. The high current can have a variety of ranges, such as greater than about 1 amp and less than about 10 amps. The calculated current can be based on the second output signal if the second output signal is greater than a threshold value, and the current is calculated is based on the first output signal if the second output signal is less than the threshold value.
In still another embodiment, the first and second voltage amplifier circuits each comprise an operational amplifier. The gain ratio between the first and second operational amplifier can be about 1:10, however, other ratios are also possible. In another embodiment, the calculated current can be a function of the measured voltage of the power source. The threshold current can be a percentage of a calibrated current, which is based on a measured voltage across the sense resistor when the light is operating correctly.
Another example of the present invention is a system for monitoring the operability of exterior lights on a vehicle. A plurality of light circuits each comprise an exterior light on the vehicle. A plurality of sense resistors each connect to a light circuit in series with the corresponding exterior light. A plurality of first operational amplifiers each connect in parallel with a sense resistor, each first operational amplifier amplifies the voltage across the corresponding sense resistor to generate a first amplified signal. One or more multiplexers are connected to the operational amplifiers to select a first amplified signal. At least one second operational amplifier is connected to the one or more multiplexers and amplifies the selected first amplified signal to generate a second amplified signal. An analog to digital converter converts the second amplified signal into a digital value. Visual indicators notify the vehicle operator if an exterior light is inoperable. A microprocessor is programmed to control the multiplexers to select a first amplified signal corresponding to an exterior light, calculate the current through the sense resistor based on the digital value corresponding to the selected first amplified signal, and compare the calculated current against a threshold current. If the calculated current is less than the threshold current the indicator corresponding to the exterior light activates.
In one embodiment, a low impedance buffer buffers the first amplified signal. In another embodiment, the second operational amplifier comprises a low current operational amplifier and a high current operational amplifier. In yet another embodiment, the calculated current is based on the second amplified signal from the low amperage operational amplifier if the light is a low current light, and the calculated current is based on the second amplified signal from the high current operational amplifier if the light is a high current light. The low current light can have a variety of ranges, such as being greater than about 0 amps and less than about 1 amp. The high current light can have a variety of ranges, such as greater than about 1 amp and less than about 10 amps. In still another embodiment, the calculated current is based on the digital value corresponding to the high current operational amplifier if such digital value is greater than a threshold value, and if such digital value is less than the threshold value then the current is calculated is based on the digital value corresponding to the low current operational amplifier.
In yet another example of the present invention, a plurality of exterior lights on the vehicle each have a corresponding light circuit and power source. A means for determining a calibrated current for each of said lights is provided. A means for measuring a voltage in each light circuit is provided where said measured voltage is proportional to the current to the corresponding light. A means for calculating the current to a selected light based on the corresponding measured voltage is provided. A means for normalizing the calculated current based on the voltage of the power source is also provided. A means determines if a light is inoperable based on the corresponding normalized calculated current and the corresponding calibrated current. A means signals the vehicle operator if light is determined to be inoperable.
Still other examples, features, aspects, embodiments, and advantages of the invention will become apparent to those skilled in the art from the following description, which is by way of illustration, one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different and obvious aspects, all without departing from the invention. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not restrictive.